1. Field of the Invention
The present invention relates to a printer device having a plurality of print heads arranged on a feeding path of paper, and more particularly to a printer device in which each print head is driven while pressing the paper against a platen.
2. Description of the Related Art
A thermal transfer color printer of a type which performs printing at relatively high speed is conventionally known. This color printer has first to fourth printing units arranged on a paper feeding path extending in one direction. The printing units are sequentially operated to print four color component images of yellow, magenta, cyan and black on the same area of paper which is fed along the paper feeding path. The four color component images are combined to form a color image. Each printing unit includes a thermal print head, a platen, an ink ribbon, and a head lift mechanism. The thermal print head is a line head having a line of heat-generating resistors disposed perpendicular to the paper feeding direction, and driven while being maintained at a contact position at which the thermal print head pressing the paper against the platen. The head lift mechanism is operated to raise the print head to a standby position apart from the contact position when the paper is fed without driving the print head. The ink ribbon is supported in such a manner that it can be fed between the heat-generating resistors and the paper in a direction perpendicular to the line of heat-generating resistors. The ink ribbon is fed at a speed corresponding to the paper feeding speed in a state where the print head is set at the contact position. A color printer of the foregoing type has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 59-188452.
Jpn. Pat. Appln. KOKAI Publication No. 8-1967 filed by the present applicant discloses a thermal transfer color printer capable of preventing a continuous paper from being wasted when issuing labels printed on the paper and separated therefrom at an outlet portion of the paper feeding path. Although this color printer is similar to the above-mentioned color printer with regard to the basic structure, the front edge of the paper is returned to an initial position from which the paper starts to be fed towards the print head of the first printing unit, except the case where a plurality of same labels are successively issued. If the print heads of the first to fourth printing units are sequentially driven to print another label without returning the front edge of the paper, the label is unintentionally printed in an area located behind a blank having a length which exceeds the distance from the print head of the first printing unit to that of the fourth printing unit. In order to prevent the paper from being wasted due to the blank, the foregoing color printer feeds the front edge of the paper backward to the initial position and then forward to load the paper into the first to fourth printing units. All the print heads are set to the standby position immediately before the paper is moved backward, and set into the contact position again immediately before the paper is moved forward.
However, the above-mentioned color printer has the following problems.
When the paper is moved forward, the print head of each printing unit is set to be in contact with the platen under pressure corresponding to the weight thereof. In this case, an impact is generated at a moment when the front edge of the paper is introduced between the print head and the platen, thereby distorting the surface of the paper. As a result, the quality of an image printed on the surface of the paper deteriorates.
The print heads of the first to fourth printing units are operated after a label printing area of the paper has reached the printing position. However, since the ink ribbons begin to be fed from the time when the print heads are simultaneously set into the contact position, the ink ribbon is wasted in a quantity corresponding to the distance from the label printing area to the corresponding print head.
In the above-mentioned color printer, each head lift mechanism includes a solenoid formed of a coil, a plunger and the like and requiring electric power continuously supplied when the paper is moved backward. The plunger is energized by an current flowing in the coil upon supply of electric power and raise the print head to the standby position against the weight of the print head. Since the foregoing operation is frequently repeated, the coil of the solenoid and the electronic components forming a driving circuit of the solenoid suffer from relatively short service life. Further, since the ink ribbon is continuously brought into contact with the paper under the weight of the print head even if the color printer is not operated, the paper can sometimes be contaminated by the ink of the ink ribbon.